Dispersion-induced structural preference in the ultrafast dynamics of diphenyl ether

Abstract

Dispersion interactions are omnipresent in large aromatic systems and influence the dynamics as intermolecular forces. The structural preference induced by dispersion interactions is demonstrated to influence the excited state dynamics of diphenyl ether (DPE) using femtosecond time-resolved transient absorption (TA) associated with quantum chemical calculations. The experimental results in aprotic solvents show that the S₁ state is populated upon irradiation at 267 nm with excess vibrational energy dissipating to solvent molecules in several picoseconds, and then decays via internal conversion (IC) within 50 ps as well as intersystem crossing (ISC) and fluorescence with a lifetime of nanoseconds. The polarity of the solvent disturbs the excited state energies and enhances the energy barriers of the ISC channel. Furthermore, the intermolecular dispersion interactions with protic solvents result in the OH-π isomer dominating in methanol and the OH-O isomer is slightly preferred in t-butanol in the ground state. The hydrogen bonded isomer measurements show an additional change from OH-O to OH-π geometry in the first 1 ps besides the relaxation processes in aprotic solvents. The time constants measured in the TA spectra suggest that the OH-O isomer facilitates IC. The results show that the OH-π isomer has a more rigid structure and a higher barrier for IC, making it harder to reach the geometric conical intersection through conformer rearrangement. This work enables us to have a good knowledge of how the structural preference induced by dispersion interactions affects excited state dynamics of the heteroaromatic compounds.

Fig. 1. Normalized steady-state absorption spectra of DPE in cyclohexane (green), 1,4-dioxane (black), methanol (red), and t-butanol (blue).

Fig. 2. Molecular orbitals of OH–O and OH–π isomers of DPE in methanol (upper) and t-butanol (bottom), respectively.

Fig. 3. TA spectra of DPE in (a) cyclohexane (b) 1,4-dioxane, (c) methanol and (d) t-butanol solvents. The bottom shows the TA spectra in (e) methanol and (f) t-butanol collected in the first 1 ps.

Fig. 4. Time traces at 650 nm obtained from the TA spectra in (a) cyclohexane (b) 1,4-dioxane, (c) methanol and (d) t-butanol solvents for delay times up to 900 ps. Open circles are data; solid black lines are the overall least-squares fit curves.

Fig. 5. The scheme of the photodeactivation pathways for the S₁ excitation in DPE. S₀: the ground state, S₁: the first excited singlet state, S_1,relax: the relaxed S₁ state, T_n: the triplet state folds, E_IC: the barrier height for IC.